A person who does not believe in god or gods.
Usually can be broken down into two groups.
1- The ones who don't believe in god, but don't try to change your mind either. They believe that people should believe, or not believe, in whatever they want. They usually have a job/friends/life.
2-The idiots who spend every second of their life on the computer, and since they do not see life outside of a computer screen, they come to the conclusion there is no god. They then, in an attempt to look cool, badmouth everything they can about religion. Most of them are teenagers looking to rebel, saying that following religion is mindless, when in fact they are following something just to be different, which is equally as mindless. They are also usually miserable bastards.
Christian: Hey, do you believe in God?
Atheist 1: Nah man, that's not for me. You wanna go to the mall?
Christian: ok, sounds awesome.
Christian:I don't know, I think theres some sort of deity watching over us.
Atheist 2: Fuck you man there is no god your a fukin retart if u think that a fukin imaginry man in the sky you fucking suck what you believe in isnt true but i know is true because im an arrogant conceited rebel
Christian:I disagree...but whatever, it's all good.
Atheist 2:yeah you hide behidn your false teaching im so fuckin smart man i read an article online that says there wasnt a god man so i listen to that because society tells me to not believ in a god and you fucking suck cocks
Atheist 1: Shut the fuck up.